The present invention relates to paper making machines in general, and more particularly to improvements in a method and apparatus for wetting continuous sheets or webs of newsprint paper or the like in paper making machines.
It is well known to increase the moisture content of continuous paper webs in the region between the drying unit and the rolling or convoluting station of a paper making machine. The means for increasing the moisture content of the web comprises a single wetting unit which employs a series of spraying devices having means for directing finely atomized liquid onto the moving web before the web is converted into a roll. Wetting of the web is desirable because the drying unit normally reduces the moisture content below that value which is most satisfactory for storage of the web in the form of a roll and/or for further processing. Moveover, uniform drying of a rapidly moving paper web (e.g., newsprint paper) is practically impossible so that the drying action is invariably regulated in such a way that the moisture content of paper leaving the drying station is too low.
The commonly owned U.S. Pat. No. 3,332,625 to Nagler et al. discloses a turbo nozzle spray damper wherein each spraying device comprises two water discharging nozzles. The air streams which issue from the air discharging nozzles are caused to spin and to peel off thin layers from the respective water jets to thus produce sprays consisting of minute water particles which impinge upon the moving paper web. The pressure of air is extremely low, normally in the range of 0.05-0.2 atmospheres superatmospheric pressure. The rate at which the liquid is sprayed onto the web can be regulated by changing the quantity of the supply of liquid in a vessel which is connected with the water discharging nozzles by discrete conduits.
The just described spray damper operates satisfactorily as long as the moisture content of the paper web which is to be wetted does not deviate appreciably from a desired optimum moisture content, as long as the speed at which the web is transported is relatively low, and as long as the web is capable of absorbing substantial quantities of water or another liquid. It has been found that, when the web is transported at a high speed (e.g., in the range of 450-900 meters per minute or higher), and when the moisture content of the freshly dried web is well below the desired mositure content, the droplets which are discharged by the patented spray damper produce readily detectable marks which detract from the appearance of the final product. Moreover, the mositure content is not uniform in all regions of the convoluted web.
The situation is further aggravated in machines for the production of newsprint paper. In such machines, the web is transported at a very high speed (within the aforementioned range) at which the drying action of air currents is normally much more pronounced at the marginal portions than in the median portion of the web. Consequently, the web which advances toward the spray damper has a low moisture content along both edges and a much higher mositure content in the central portion thereof. The differences between the moisture content of the marginal portions and the mositure content of the median portion become more pronounced if the speed of lengthwise movement of the web is increased. For example, when a web of newsprint paper is transported at a speed of up to and in excess of 900 meters per minute, the mositure content of the median portion often exceeds the mositure content of the marginal portions by as much as 3-4 percent. This is undesirable for a number of reasons. Attempts to eliminate such pronounced differences in moisture content have met with limited success. As a rule, an equalization of moisture content with presently known wetting apparatus results in the formation of readily detectable spots which develop where the particles of liquid impinge on the web while the latter travels past the spray damper.